Al—Mg-based JIS 5000 series aluminum alloy sheet is excellent in strength and press-formability, so is being studied for application to automobile body panels etc. as an alternative material to steel sheet. To shape it to the desired form, it has to be shaped by a press die. 5000 series aluminum alloy sheet excellent in so-called press-formability has been developed. However, 5000 series aluminum alloy sheet is inferior to steel sheet in press-formability, so measures such as dividing parts into pieces for press-forming become necessary. The problem of the increase in number of parts and number of dies consequently arises. In particular, in press-forming of complicated shapes, cases are often seen of cracking and other defects occurring near the plane strain region.
Further improvement of the so-called plane strain fracture limit is therefore becoming a hot issue.
For example, PLT 1 describes aluminum alloy sheet for shaping use excellent in mechanical properties characterized by containing, by weight %, Mg: 2.0% to 6.0%, Si: 1.5% or less, and Fe: 1.5% or less, having a balance of Al and unavoidable impurities, and having remaining intermetallic compounds of an average size of 15 μm or less. According to this, if making the thickness of the slab which is cast 1 to 10 mm or so, the average size of the intermetallic compounds can be controlled to 15 μm or less and aluminum alloy sheet which is excellent in formability can be obtained.
On the other hand, PLT 2 discloses Al—Mg-based alloy sheet which has a texture with a volume fraction of the CUBE orientation of 30% to 50% and a volume fraction of the BRASS orientation of 10% to 20% and which has a grain size of 50 to 100 μm in range. According to this, by controlling the ratio of the individual crystal orientations for the texture, which governs the plastic anisotropy of aluminum alloy sheet, and by further optimizing the grain size and further by limiting the types and added amounts of the added elements, it is possible to obtain aluminum alloy sheet which is excellent in press-formability.
Recently, high strength aluminum alloy sheet which is provided with both the excellent skin roughness and formability which are suitable for structural materials of household electrical appliance products or automobile panels etc. and methods of production of the same have been proposed. PLT 3 describes high strength aluminum alloy sheet which has a chemical composition which contains Mg: 2.0 to 3.3 mass %, Mn: 0.1 to 0.5 mass %, and Fe: 0.2 to 1.0 mass %, has a balance of unavoidable impurities and Al, and, in the unavoidable impurities, has Si: less than 0.20 mass %, has an average circle equivalent diameter of intermetallic compounds of 1 μm or less, has an area rate of intermetallic compounds of 1.2% or more, and has an average size of recrystallized grains of 10 μm or less and a method of production of the same. According to this, Fe, in copresence with Mn and Si, causes the precipitation of fine Al—(Fe.Mn)—Si-based compounds at the time of casting, raises the strength, and improves the formability.